The free homeowner guide: where the valves are and what to do in the first five minutes.
Usually at the front of the house near the hose bib, or in the garage on the wall facing the street. Turns water off to the whole home. Turn it clockwise (or a quarter turn on a lever handle) until it stops.
In the meter box at the curb. This is the city's, and it is the backup if your main valve is stuck or broken. A meter key or wrench is usually needed.
Under every sink and behind every toilet, plus the washer box and the water heater supply. These stop one problem without shutting down the house.